Sodium-ion batteries are analogous in many ways to the lithium-ion batteries that are in common use today; they are both reusable secondary batteries that comprise an anode (negative electrode), a cathode (positive electrode) and an electrolyte material, both are capable of storing energy, and they both charge and discharge via a similar reaction mechanism. When a sodium-ion (or lithium-ion battery) is charging, Na+ (or Li+) ions de-intercalate from the cathode and insert into the anode. Meanwhile charge balancing electrons pass from the cathode through the external circuit containing the charger and into the anode of the battery. During discharge the same process occurs but in the opposite direction.
Lithium-ion battery technology has enjoyed a lot of attention in recent years and provides the preferred portable battery for most electronic devices in use today; however lithium is not a cheap metal to source and is considered too expensive for use in large scale applications. By contrast sodium-ion battery technology is still in its relative infancy but is seen as advantageous; sodium is much more abundant than lithium and some researchers predict this will provide a cheaper and more durable way to store energy into the future, particularly for large scale applications such as storing energy on the electrical grid. Nevertheless a lot of work has yet to be done before sodium-ion batteries are a commercial reality.
NaNi0.5Mn0.5O2 is a known Na-ion material in which the nickel is present as Ni2+ while the manganese is present as Mn4+. The material is ordered with the Na and Ni atoms residing in discrete sites within the structure. The nickel ions (Ni2+) are a redox element which contributes to the reversible specific capacity and the manganese ions (Mn4+) play the role of a structure stabilizer. Compound NaNi0.5Ti0.5O2 is analogous to NaNi0.5Mn0.5O2 in that the Ni2+ ions provide the active redox centre and the Ti4+ ions are present for structure stabilization. There is plenty of literature describing the preparation of NaNi0.5Mn0.5O2 (and to a lesser extent NaNi0.5Ti0.5O2) as the precursor for making LiNi0.5Mn0.5O2 and LiNi0.5Ti0.5O2 by Na→Li ion exchange for Li-ion applications. A direct synthesis method to make these Li materials yields undesirable disordered materials, for example, as a result of the lithium and nickel atoms sharing structural sites.
Recent electrochemical studies reported by Komaba et al Adv. Funct. Mater. 2011, 21, 3859 describe the sodium insertion performance of hard-carbon and layered NaNi0.5Mn0.5O2 electrodes in propylene carbonate electrolyte solutions. The results obtained show that although NaNi0.5Mn0.5O2 exhibits some reversible charging and discharging ability, the capacity of the material fades by 25% or more, after only 40 cycles.
Kim, Kang et al, Adv. Energy Mater. 2011, 1, 33-336 also discusses the preparation of a layered oxide material with a structure, normalised to 2O2−, of Na0.85Li0.17Ni0.21Mn0.64O2 and its use to reversibly intercalate sodium in a sodium cathode material. However, as this paper describes, this material only demonstrates modest electrochemical performance.
The present invention aims to provide novel compounds. Further, the present invention aims to provide a cost effective electrode that contains an active material that is straightforward to manufacture. Another aim of the present invention is to provide an electrode that has a high initial specific discharge capacity and which is capable of being recharged multiple times without significant loss in charge capacity.
Therefore, the first aspect of the present invention provides compounds of the formula:AUM1VM2WM3XM4YM5ZO2                 wherein        A comprises one or more alkali metals selected from lithium, sodium and potassium;        M1 is nickel in oxidation state +2        M2 comprises a metal in oxidation state +4 selected from one or more of manganese, titanium and zirconium;        M3 comprises a metal in oxidation state +2, selected from one or more of magnesium, calcium, copper, zinc and cobalt;        M4 comprises a metal in oxidation state +4, selected from one or more of titanium, manganese and zirconium;        M5 comprises a metal in oxidation state +3, selected from one or more of aluminium, iron, cobalt, molybdenum, chromium, vanadium, scandium and yttrium;        further wherein        U is in the range 1<U<2;        V is in the range 0.25<V<1;        W is in the range 0<W<0.75;        X is in the range 0≦X<0.5;        Y is in the range 0≦Y<0.5;        Z is in the range 0≦Z<0.5;        and further wherein (U+V+W+X+Y+Z)≦3.        Preferably (U+V+W+X+Y+Z)≦2.5.        
Although A is defined as comprising one or more alkali metals selected from lithium, sodium and potassium, compounds in which A is one or more alkali metal comprising sodium and/or potassium either alone or in a mixture with lithium as a minor constituent are also part of the present invention.
Preferably the present invention provides a compound of the above formula wherein U is in the range 1<U<1.5; V is in the range 0.25<V<1, preferably 0.3<V<1; W is in the range 0<W<0.75; X is in the range 0≦X≦0.25; Y is in the range 0≦Y≦0.25; Z is in the range 0≦Z≦0.25.                Particularly preferred compounds of the above formula include:        Na1.1Ni0.35Mn0.55O2;        Na1.05Ni0.425Mn0.525O2;        Li1.1Ni0.35Mn0.55O2;        Li1.05Ni0.425Mn0.525O2;        Na1.1Ni0.3Mn0.5Al0.1O2;        Na1.05Ni0.4Mn0.5Al0.05O2;        Li1.1Ni0.3Mn0.5Al0.1O2;        Li1.05Ni0.4Mn0.5Al0.05O2;        Na1.1Ni0.3Mn0.5Mg0.05Ti0.05O2;        Na1.05Ni0.4Mn0.5Mg0.025Ti0.025O2;        Na1.05Ni0.4Mn0.5Mg0.025Ti0.025O2;        Na1.5Ni0.75Mn0.25O2;        Na1.5Ni0.75Ti0.25O2;        Li1.5Ni0.75Mn0.25O2;        Li1.5Ni0.75Ti0.25O2;        Na1.1Ni0.3Ti0.05Mg0.05Mn0.5O2;        Na1.05Ni0.4Ti0.025Mg0.025Mn0.5O2;        Li1.1Ni0.3Ti0.05Mg0.05Mn0.5O2;        Li1.05Ni0.4Ti0.025Mg0.025Mn0.5O2.        
In a second aspect, the present invention provides an electrode comprising an active compound of the formula:AUM1VM2WM3XM4YM5ZO2                 wherein        A comprises one or more alkali metals selected from lithium, sodium and potassium;        M1 is nickel in oxidation state +2        M2 comprises a metal in oxidation state +4 selected from one or more of manganese, titanium and zirconium;        M3 comprises a metal in oxidation state +2, selected from one or more of magnesium, calcium, copper, zinc and cobalt;        M4 comprises a metal in oxidation state +4, selected from one or more of titanium, manganese and zirconium;        M5 comprises a metal in oxidation state +3, selected from one or more of aluminium, iron, cobalt, molybdenum, chromium, vanadium, scandium and yttrium;        further wherein        U is in the range 1<U<2;        V is in the range 0.25<V<1;        W is in the range 0<W<0.75;        X is in the range 0≦X<0.5;        Y is in the range 0≦Y<0.5;        Z is in the range 0≦Z<0.5;        and further wherein (U+V+W+X+Y+Z)≦3.        
Preferably (U+V+W+X+Y+Z)≦2.5.
Although A is defined as comprising one or more alkali metals selected from lithium, sodium and potassium, compounds in which A is one or more alkali metals comprising sodium and/or potassium either alone or in a mixture with lithium as a minor constituent are also part of the present invention.
Preferably the present invention provides an electrode comprising an active compound of the above formula wherein U is in the range 1<U<1.5; V is in the range 0.25<V<1, preferably 0.3<V<1; W is in the range 0<W<0.75; X is in the range 0≦X≦0.25; Y is in the range 0≦Y≦0.25; Z is in the range 0≦Z≦0.25.
Especially preferred electrodes comprise active compounds selected from one or more of:                Na1.1Ni0.35Mn0.55O2;        Na1.05Ni0.425Mn0.525O2;        Li1.1Ni0.35Mn0.55O2;        Li1.05Ni0.425Mn0.525O2;        Na1.1Ni0.3Mn0.5Al0.1O2;        Na1.05Ni0.4Mn0.5Al0.05O2;        Li1.1Ni0.3Mn0.5Al0.1O2;        Li1.05Ni0.4Mn0.5Al0.05O2;        Na1.1Ni0.3Mn0.5Mg0.05Ti0.05O2;        Na1.05Ni0.4Mn0.5Mg0.025Ti0.025O2;        Na1.05Ni0.4Mn0.5Mg0.025Ti0.025O2;        Na1.5Ni0.75Mn0.25O2;        Na1.5Ni0.75Ti0.25O2;        Li1.5Ni0.75Mn0.25O2;        Li1.5Ni0.75Ti0.25O2;        Na1.1Ni0.3Ti0.05Mg0.05Mn0.5O2;        Na1.05Ni0.4Ti0.025Mg0.025Mn0.5O2;        Li1.1Ni0.3Ti0.05Mg0.05Mn0.5O2; and        Li1.05Ni0.4Ti0.025Mg0.025Mn0.5O2.        
The electrodes according to the present invention are suitable for use in many different applications, for example energy storage devices, rechargeable batteries, electrochemical devices and electrochromic devices.
Advantageously, the electrodes according to the invention are used in conjunction with a counter electrode and one or more electrolyte materials. The electrolyte materials may be any conventional or known materials and may comprise either aqueous electrolyte(s) or non-aqueous electrolyte(s) or mixtures thereof.
In a third aspect, the present invention provides an energy storage device that utilises an electrode comprising the active materials described above, and particularly an energy storage device for use as one or more of the following: a sodium ion and/or lithium ion and/or potassium ion cell; a sodium metal and/or lithium metal and/or potassium metal ion cell; a non-aqueous electrolyte sodium ion and/or lithium ion and/or potassium ion cell; an aqueous electrolyte sodium ion and/or lithium ion and/or potassium ion cell.
The novel compounds of the present invention may be prepared using any known and/or convenient method. For example, the precursor materials may be heated in a furnace so as to facilitate a solid state reaction process.
A fourth aspect of the present invention provides a particularly advantageous method for the preparation of the compounds described above comprising the steps of:
a) mixing the starting materials together, preferably intimately mixing the starting materials together and further preferably pressing the mixed starting materials into a pellet;
b) heating the mixed starting materials in a furnace at a temperature of between 400° C. and 1500° C., preferably a temperature of between 500° C. and 1200° C., for between 2 and 20 hours; and
c) allowing the reaction product to cool.
Preferably the reaction is conducted under an atmosphere of ambient air, and alternatively under an inert gas.
It is also possible to prepare lithium-ion materials from the sodium-ion derivatives by converting the sodium-ion materials into lithium-ion materials using an ion exchange process.
Typical ways to achieve Na to Li ion exchange include:
1. Mixing the sodium-ion rich material with an excess of a lithium-ion material e.g. LiNO3, heating to above the melting point of LiNO3 (264° C.), cooling and then washing to remove the excess LiNO3 and side reaction products;
2. Treating the Na-ion rich material with an aqueous solution of lithium salts, for example 1M LiCl in water; and
3. Treating the Na-ion rich material with a non-aqueous solution of lithium salts, for example LiBr in one or more aliphatic alcohols such as hexanol, propanol etc.